You will gain more units as characters join your army, and some of them are actually quite workable!
Also, here's somehting I wish I had thought of for the first few missions:
Enemy units generally don't retreat unless they are heavily wounded with at least a few dead members, or if their leader is dead. With your battle strategy set to Strong, you increase your chances of wiping out enem units altogether, though it will take more battles to do so.
In case you don't know how, you can set your enemy Tactics, Retreat, or use Tarot cards (all of which are things noname outlined before) by pressing A during battle! You will hear a beeping sound if the press registered (if it doesn't, just keep pressing!) and then you will be able to select what you want to do. The battle freezes while you do this, so no rush.
Interesting fact:
If you select Strong as your strategy, you will both inflict and take less damage.
If you select Weak as your strategy, you will both inflict and take more damage.
Strong may be the best strategy to use early on, not only because it allows you to take down enemy units more decisively but also because it will make your own units survive for longer!
Note, though, that this is not the case for boss fights. In boss fights you should always set your strategy to Leader, because beating the enemy unit's leader in a boss fight will win the level!